Structure and method of forming semiconductor device

1. A method comprising: forming a first dielectric layer over a substrate; forming a gate structure over a first portion of the first dielectric layer, wherein the gate structure includes a gate electrode; forming a sidewall spacer over a second portion of the first dielectric layer and on the gate structure; converting the second portion of the first dielectric layer and an exposed third portion of the first dielectric layer to a first portion of a second dielectric layer and a second portion of the second dielectric layer, respectively, wherein the second dielectric layer is more etch resistant than the first dielectric layer; removing the second portion of the second dielectric layer and a portion of the substrate to form a recess in the substrate adjacent the sidewall spacer; forming a source/drain (S/D) feature in the recess; and removing the gate electrode and the first portion of the first dielectric layer to form a gate trench, wherein the first portion of the second dielectric layer prevents lateral etch towards the S/D feature during the removing of the gate electrode and the first portion of the first dielectric layer.

2. The method of claim 1 , wherein converting the second portion of the first dielectric layer and the exposed third portion of the first dielectric layer to the first portion of the second dielectric layer and the second portion of the second dielectric layer, respectively, includes performing an implantation process that incorporates a dopant into the second and third portions of the first dielectric layer.

3. The method of claim 2 , wherein the first portion of the first dielectric layer remains intact during the ion implantation.

4. The method of claim 2 , further comprising, applying the ion implantation with a species including one or more from the group consisting of carbon, boron, indium, silicon, nitrogen and helium.

5. The method of claim 4 , further comprising, applying a carbon implantation to the second and third portion of the silicon oxide layer to convert them to carbon-containing silicon oxide layer.

6. The method of claim 2 , further comprising, applying the ion implantation with a tilt angle to convert the second portion of the first dielectric layer into the second dielectric layer.

7. The method of claim 1 , further comprising: prior to forming the S/D feature in the S/D recess, applying a wet etching process to remove native oxide in the S/D recess.

8. The method of claim 7 , wherein the second dielectric layer is formed to have a higher etching resistance to the wet etching process than the first dielectric layer.

9. The method of claim 1 , further comprising: forming a high-k/metal gate in the gate trench.

10. The method of claim 1 , wherein the first dielectric layer includes silicon oxide layer.

11. A method for forming a semiconductor device comprises: providing a silicon oxide layer interposed between a substrate and a dummy gate stack having a gate spacer along sidewalls, the silicon oxide layer has three portions: a first portion which is underneath the dummy gate stack; a second portion which is underneath the gate spacer; and a third portion which is outside of the gate spacer and the dummy gate stack; applying an ion implantation to the second and third portions of the silicon oxide layer to convert them to a dopant-containing silicon oxide layer to increase an etching resistance of the dopant-containing silicon oxide layer to an oxide etching process; after receiving the ion implantation, etching the third portion of the silicon oxide layer and extending to etch the substrate underneath of it to form a source/drain (S/D) recess; forming a source/drain (S/D) feature in the S/D recess and continually forming the S/D feature above the substrate; and removing the dummy gate stack and the first portion of the silicon oxide layer, which does not receive the ion implantation, with a high etch selectivity to the second portion of the silicon oxide, which receives the ion implantation, to form a gate trench.

12. The method of claim 11 , wherein the first portion of the silicon oxide layer remains intact during the ion implantation.

13. The method of claim 11 , further comprising, applying the ion implantation with a species including one or more from the group consisting of carbon, boron, indium, silicon, nitrogen, helium.

14. The method of claim 13 , further comprising, applying a carbon implantation to the second and third portion of the silicon oxide layer to convert them to carbon-containing silicon oxide layer.

15. The method of claim 11 , further comprising, applying the ion implantation with a tilt angle to ion implant the second portion of the silicon oxide layer.

16. The method of claim 11 , further comprising: prior to forming the S/D feature, applying a wet etching process to remove native oxide in the S/D recess.

17. The method of claim 16 , wherein after receiving the ion implantation, the second portion of the silicon oxide layer has a higher etching resistance to the wet etching process than before receiving the ion implantation.

18. The method of claim 11 , further comprising: forming a high-k/metal gate in the gate trench.

19. A device comprising: a high-k/metal gate (HK/MG) over a substrate, the HK/MG comprising a first dielectric layer and an electrode disposed over the first dielectric layer; a spacer along a sidewall of the HK/MG; a second dielectric layer disposed between the spacer and the substrate directly below the spacer, wherein the first dielectric layer of the HK/MG physically contacts the second dielectric layer, the second dielectric layer being more etch resistant than the first dielectric layer; and a source/drain (S/D) feature beside of the HK/MG, wherein an upper portion of the S/D feature above the substrate and a lower portion of the S/D feature embedded in the substrate, wherein the second dielectric layer is disposed between the upper portion of the S/D feature and the HK/MG.

20. The device of claim 19 , wherein the second dielectric layer includes silicon oxide with ion implanted species including one or more from the group consisting of carbon, boron, indium, silicon, nitrogen, helium.